Method and device for measuring the distance between the discs of a refiner using a measurement of the magnetic flux induced between the discs

ABSTRACT

This invention is related to a method and device for measuring the distance (δ) between the discs of a refiner, in which method a magnetic field is generated by means of a coil (5) placed at one disc (4), which magnetic field is allowed to run at least partly through the other disc (3). According to the invention the measuring is carried out by using the actual distance between the refiner discs as the object of measuring in that the magnetic flux over the distance between the discs caused by the coil (5), which is formed at least at one refiner-disc tooth (7) or comparable element, is measured such that the flux running over the distance between two opposite teeth (7,8) is detected by means of another coil (6), such as is formed round at least either of said teeth, the signal induced in the coil being interpreted as a quantity that expresses the distance (δ) between the teeth.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention is related to a method and device for measuring thedistance between the discs of a refiner, in which method a magneticfield is generated by means of a coil placed at one disc, which magneticfield is allowed to run at least partly through the other disc.

2. The Prior Art

Measuring the distance between, for instance, the discs of a discrefiner has chiefly two purposes. Firstly, the aim is to prevent thediscs from accidentally getting closer to each other and finallycolliding, which is caused by the yielding of the refiner, its wearingand other factors, because this way the valuable discs wear quickly andmight also easily get destroyed altogether. Secondly, measuring thedistance between the discs provides valuable information about theprocess itself, and it is possible to see, for instance, what distancebetween the discs gives the desired final result and adjust the distanceaccording to the quality of the final product already at the beginningof the process. This way costly experiments are avoided.

In each case the equipment for measuring the distance between the discshas to meet high requirements because it has to work under greatpressure and reliably measure distances no bigger than fractions of amillimeter in a vibrating machine. Known devices for measuring thedistance between the discs are based either on indirect measuring, or onthe use of sensors for detecting the distance between the discs, or oncapacitative measuring.

On example of indirect measuring is the measuring of the axial movementof the discs. The axial movement of the discs is in principleproportional to changes in the distance between the discs in a discrefiner, and the same rule applies also to a cone refiner. The yieldingof the refiner under varying forces and pressures and the wearing of allparts between the discs' mutual distance and the indirect measuringpoint, however, make the results of the measurement unreliable. Besides,this kind of measuring does not provide information about the wearing ofthe disc surfaces themselves, in other words their condition.

Another way to measure the distance between the discs is to use built-inapproach sensors which, installed in one disc, measure the distancebetween the opposite disc and themselves, as described in Finnish patentapplication No. 801748. Both in the invention presented by means of thatpatent application and in the description of prior art, the point ofdeparture is the measuring of the distance between smooth surfaces,which measuring is problematic. In the case of said invention furtherproblems are the discs' wearing and the resultant changes in theirmutual distance. As the sensors do not wear in the same way as the discmaterial, if at all, they are certainly not able to take intoconsideration the wearing of the disc in which they are mounted. A thirdway of measuring referred to is to measure the capacitance between thediscs and its changes, on the basis of which, for instance, in a discrefiner the average distance between the rotor disc and the stator disccan be determined. yet the capacitance measured is also influenced byseveral other factors, such as the chemical properties of the woodpulpto be ground and of the dilution water. Thus the results are very easilydistorted; consequently, no practical applications of this method areknown to be in the market. Besides, it is impossible to measure theangle between the discs with this method only.

In addition to the above-mentioned problems, all above-mentioned methodsare burdened by difficulties of calibration because the absolutelocation of the measuring points needed for calibration or theirlocation in relation to the discs is constantly changing as time goeson.

SUMMARY OF THE INVENTION

The object of this invention is to create a method and device formeasuring the distance between the discs of a refiner without theabove-mentioned drawbacks. To achieve this, a method according to theinvention is characterized by carrying out the measuring by using theactual distance between the refiner discs as the object of measuring inthat the magnetic flux over the distance between the discs caused by thecoil, which is formed at least at one refiner-disc tooth or comparableelement, is measured by that the flux running over the distance betweentwo opposite teeth is detected by means of a coil, such as is formedround at least either of said teeth, the signal induced in the coilbeing interpreted as a quantity that expresses the distance between theteeth.

The most significant advantages of the invention are the following:

the actual critical parts of the discs are used for the measuring of thedistance between the discs, which gives the real value irrespective ofthe discs' wearing and the refiner's yieldings. The accurate measuringmethod also creates opportunities for a soundly utilizable multi-pointmeasuring, with which the obliquity of the positions of the discs inrespect to each other can be determined.

It is to be noted that because the measuring in the invention is basedon the measuring of a magnetic field the materials between the discs(water, wood chips), being magnetically not conductive, do not influencethe results.

An advantageous application of the invention is characterized by feedinga three-wave, that is sawtooth voltage into the coil that generates themagnetic field.

From this kind of amplitude of an easily modifiable signal it ispossible to read the distance between the discs as a quantity reverselyproportial to the amplitude.

Another advantageous application of the invention is characterized bycarrying out the calibration of the measuring device by measuring thecorresponding signal levels with the discs in the extreme positions inrelation to each other and by recording said signal levels into amemory.

the difficulty of calibration which burdens devices based on prior artare eliminated from a solution provided by the invention partly throughthe absolute method of measurement and partly through the utilization ofsuch electronic equipment as is in agreement with the applicationdescribed above.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following, the invention is explained in more detail by means ofan example with reference to the enclosed drawings, in which:

FIG. 1 shows an advantageous application of the invention.

FIG. 2 shows the field lines of the most important routes of themagnetic flux in a case like that in FIG. 1.

FIG. 3 shows another advantageous application of the invention.

FIG. 4 shows as a block diagram the control electronics of a deviceaccording to the invention.

FIG. 5 shows the position of a coil according to the invention at a discof a disc refiner as seen from line v--v of FIG. 1.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows a cross-section along the outer circles of the discs of adisc refiner. In a situation like that in the figure, the teeth of thetwo discs are positioned opposite each other. The base plates of therotor and of the stator are denoted with numbers 1 and 2 respectively.Likewise, the rotor and stator discs are denoted with numbers 3 and 4respectively. In one disc, in this case the stator disc 4, cavities 7aand 7b have been machined surrounding one tooth 7. Round the tooth 7 hasthen been formed a coil 5, and into that coil is fed sawtooth voltage.The coil compels the magnetic flux caused by it to run as effectively aspossible through the magnetic body in the field, in this case therefiner-disc tooth 7 in the coil.

If the wearing properties of the base plates 1,2 of the rotor and of thestator respectively are close enough to the wearing properties of theactual disc material, the coils can also be placed at the locations A orB of the base plates, which form the outer circles of the respectiverefiner discs. This has the advantage that the coils are readilyaccessible for instance for maintenance.

In FIG. 2 dashed lines show how the magnetic flux runs in a case likethat in FIG. 1. It is seen, that there arises at every tooth a closedmagnetic field which because of the shape of the discs has an air gap.The total flux runs through the tooth that is equipped with the coil.The flux returns through all the other teeth in the magnetic circuit, ofwhich the disc has a great number. The coil 5 corresponds to the primarycoil of an iron-cored transformer and the flux induced by it creates avoltage in the other coil 6, which in turn corresponds to the secondarycoil of a transformer. For reasons explained later on, the coils 5, 6 donot, however, operate as a transformer. The coils 5, 6 can be one andthe same coil.

Magnetic flux φ is proportional to the permeance of the magneticcircuit: ##EQU1## in which N=number of revolutions of the primary coil

i=current of the coil

μo=permeability of free space, defined as 4π10⁻⁷ H/M

A=area of the material permeated by the flux

δ=(delta) distance between the discs (in this case)

P=permeance.

In a circuit like that in FIG. 1 the permeance of the circuit can bedivided into two components: ##EQU2## in which A=area of onerefiner-disc tooth.

the contribution of iron has been left out of the equation. Thecomponents represent the permeance of the air gap permeated by the mainflux and the permeance of the air gaps of the flux that returns throughthe other teeth (n pcs). As permeance is the inverse of reluctance, weget ##EQU3## from which follows that ##EQU4## when n→∞

Thus, according to this only the air gap at the coil tooth 7 isdecisive.

The flux received by the coil 6 is reversely proportional to thedistance δ between the refiner-disc teeth 7 and 8 because an increase inthe distance also means an increase in magnetic reluctance, in otherwords in the resistance in the closed circuit where the magnetic fluxruns. The invention is based on the very idea that utilization ofrefiner-disc teeth for creating a magnetic circuit and the detection ata tooth 7 itself of the flux changes in one such magnetic circuitprovide such a field intensity and precision as can be measured withoutthe drawbacks that occur when measuring the distance between planarsurfaces. Besides, what is measured all the time is the actual distancebetween the discs, since the changes in the distance between therefiner-disc teeth are directly reflected in the measurements,irrespective of why the changes occur (inner yieldings of the refiner,wearing).

when the rotor of the refiner rotates in relation to the stator, themagnetic flux naturally changes cyclically because the magneticresistance between the teeth, in other words reluctance, is at itslowest when the teeth are precisely opposite each other, at which timethe flux correspondingly is at its greatest. This maximum value is theactual result of the measurement, and it can be easily discerned forinstance by filtering or with a peak-value detector.

FIG. 3 shows another application of the invention, in which a winding 9is installed longitudinally in a groove between two refiner-disc teeth.The winding causes the same kind of magnetic circuit as in the formercase, as is shown by the flux line 10. This flux can be detected andmeasured in the same way as that shown in FIG. 1. As a new feature inFIG. 3 is a so-called short-circuit coil 11, with which the flux isprevented from running outside its area of generation. Because of theflux, in the coil 11 is induced a current which in turn reduces themagnetic field that runs through the coil. It is furthermore seen inFIG. 3 that the discs can also consist of separate segments 12, 13, forinstance such as can be changed one by one.

FIG. 4 shows the control electronics of a system according to theinvention for measuring the distance between the discs. As alreadystated above, into the generating coils 5,6 is fed sawtooth voltage.This is because the disc material is not a particularly goodferromagnetic material. Among other things, therefore, this means thatit cannot be made crystal-oriented, which is what, for instance,cold-rolled transformer plates mostly are. Owing to this and with anappropriate number of windings of the coils, the flux, in practice,tends to get quickly saturated, which causes the magnetic field tobehave derivatively. Thus in this kind of situation, for instance, thecoils 5 and 6 and the disc 4 become a derivator, not a transformer. If,in that case, the supply voltage is sawtoothed, the curve form of theoutput signal of the secondary side will advantageously, for furtherprocessing, be that of a square wave, the amplitude of which isreversely proportional to the distance δ between the discs. As statedearlier, is is possible at the same time to both generate and detect amagnetic field with the coil 5, in which case a separate coil 6 is notnecessary. The circuit shown in Figure 4 is not essentially changed bythis.

In FIG. 4 a sawtooth voltage generator 14 feeds three-wave voltage intothe coil 5. The output signal of the secondary coil 6 of the transformercircuit is, as stated earlier, square wave, and it is rectified in arectifier 15. The rectified signal U₁ is taken into a reduction element16, in which the signal U₂, the stray flux of the magnetic field and thebackground noise of the rest of the circuit, is subtracted from thesignal U₁, the result being signal U₃. Finally, this signal is correctedin the circuit 17 by means of a coefficient chosen on the basis of thedisc material and design, in such a way that the output of the circuitdirectly represents the distance δ between the discs. Said coefficientalso takes into consideration the dislinearity of the interdependencebetween the value δ of the discs' distance from each other and theamplitude of the corresponding square wave, in such a way that theoutput δ value is in a direct and linear way dependent on the distancebetween the discs, which makes eventual further processing as easy aspossible. The output signal can be binary or analog. In practice, thecircuit 17 can comprise a microprocessor, the advantages of which areeasy modifiability of the corrective coefficients and a wide range ofpossibilities of further processing.

The definition of the background noise U₂ and at the same time thecalibration of the device happen automatically (no scaling or searchingfor reference points), in such a way that the discs of the refiner aremoved far from each other, for instance to the maintenance position, andthe device is allowed to measure the signal that corresponds to this`indefinite` distance between the discs. The resultant signal, in otherwords the base voltage U₂, which mainly represents the stray flux of themagnetic field, is recorded into a holding circuit 18, so that it inreal measurement could be subtracted from the measured signal. Finally,in the calibration the discs are moved together to zero distance, bywhich means the circuit is informed about the zero position signalcharacteristic of the disc type. It is thereby possible to form andcheck the correction coefficient stored in the circuit 17.

This way, a device according to the invention can be advantageouslyscaled and calibrated; and because the measuring method is insensitiveto the reasons of the changes in the the discs' distance from each otherand to the quality and amount of the material between the discs, thecalibration generally need not be carried out more than once, wheninstalling the discs. Finally FIG. 5 shows a cross-sectional view takenalong line V--V of FIG. 1 of a real situation where a port of a discsection 19 has been provided around a separate refiner-disc tooth 22with machined grooves 20 and fastening places 21 for the coils. Thecoils are fastened by screwing through points 21 screws that go throughthe disc, after which the coils are covered by casting a solid plasticlayer over them.

It is obvious to a person skilled in the art that the invention is notrestricted to the embodiments described above, but can be varied withinthe scope of the following patent claims.

We claim:
 1. A method for measuring the distance δ between discs in a refiner, each of said discs having at least one tooth, said method comprising:generating a magnetic field by means of first coil means formed at least at one refiner-disc tooth of a first disc, said magnetic field being allowed to run at least partly through a second disc, the second disc comprising a tooth; detecting, by means of a second, coil means formed around the tooth of the first disc, the magnetic flux caused by the first coil means running over the distance between two opposite teeth of the first and second discs, respectively; outputting a signal representing the detected magnetic flux; and interpreting the signal as a quantity that expresses distance δ between the teeth.
 2. The method according to claim 1, wherein the step of generating comprises the step of generating the magnetic field by a first coil wound around one disc tooth along at least part of its length and the step of detecting comprises the step of detecting the magnetic flux by a separate second coil wound around the one disc tooth.
 3. The method according to claim 1, wherein the step of generating comprises the step of generating the magnetic field by a coil which is wound to run through a groove between two teeth and through the rear side of the disc.
 4. The method according to claim 1, wherein the step of generating comprises the step of generating the magnetic field by a first coil wound around one disc tooth along at least part of its length and the step of detecting comprises the step of detecting the magnetic flux by a separate second coil wound around the same disc tooth.
 5. The method according to claim 1, wherein the steps of generating and detecting respectively comprise the steps of generating and detecting the magnetic field by separate coils which are wound around the same tooth along at least part of its length.
 6. The method according to claim 1, wherein the step of generating comprises the step of directing the course of the magnetic field generated by the magnetic coil with short-circuit coils installed at the disc.
 7. The method according to claim 1, further comprising the step of performing the above-listed steps of generating, detecting, outputting, and interpreting the distance δ between the discs of the refiner at more than one location.
 8. The method according to claim 1, wherein the step of generating comprises the step of feeding a saw-tooth voltage into the first coil means that generates the magnetic field.
 9. The method according to claim 1, wherein the step of outputting comprises the step of measuring, at the output of the second coil means that detects the magnetic field, the amplitude (U₁) of a voltage signal that has the shape of a square wave.
 10. The method according to claim 1, further comprising the step of calibrating the measuring device comprising the steps of measuring the corresponding signal levels with the discs in the extreme positions in relation to each other and recording said signal levels into a memory.
 11. A device for the implementation of claim 1, said device comprises:a first coil for the generation of a magnetic field, said first coil being wound at one refiner-disc tooth of one refiner disc; a second coil for the detection of the flux running over the distance between two opposite teeth wound to surround at least partly said refiner-disc tooth of said first coil; an electronic circuit for the interpretation of the signal induced in said first coil.
 12. The device according to claim 11, wherein the first coil that generates the magnetic field and the second coil that detects the magnetic field are wound around the one refiner-disc tooth along at least part of its length.
 13. The device according to claim 11, wherein the first coil that generates the magnetic field is wound to run through a groove between two teeth and through the rear side of the disc.
 14. The device according to claim 11, wherein the first coil that generates the magnetic field and the second coil that detects the magnetic field are wound around the same refiner-disc tooth along at least part of its length.
 15. The device according to claim 11, wherein the first coil that generates the magnetic field and the second coil that detects the magnetic field are wound around the same tooth of either disc along at least part of its length.
 16. The device according to claim 11, further comprising short-circuit coils installed at the refiner disc that direct the course of the magnetic field.
 17. The device according to claim 11, further comprising several pairs of coils installed at the refiner disc that measure the distance (δ) between the discs of the refiner at more than one location.
 18. The device according to claim 11, further comprising a saw-tooth voltage generator for providing voltage to the first coil that generates the magnetic field.
 19. The device according to claim 11, wherein the electronic circuit comprises a rectifier, a reduction circuit, a modifying circuit, and a holding circuit for the background noise. 